Carlos G. Camara

Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape

Relative motion between two contacting surfaces can produce visible light, called triboluminescence. This concentration of diffuse mechanical energy into electromagnetic radiation has previously been observed to extend even to X-ray energies. Here we report that peeling common adhesive tape in a moderate vacuum produces radio and visible emission, along with nanosecond, 100-mW X-ray pulses that are correlated with stick–slip peeling events. For the observed 15-keV peak in X-ray energy, various models give a competing picture of the discharge process, with the length of the gap between the separating faces of the tape being 30 or 300 μm at the moment of emission. The intensity of X-ray triboluminescence allowed us to use it as a source for X-ray imaging. The limits on energies and flash widths that can be achieved are beyond current theories of tribology.

Sonoluminescence: nature’s smallest blackbody

The transduction of sound into light through the implosion of a bubble of gas leads to a flash of light whose duration is delineated in picoseconds. Combined measurements of spectral irradiance, Mie scattering, and flash width (as determined by time-correlated single-photon counting) suggest that sonoluminescence from hydrogen and noble-gas bubbles is radiation from a blackbody with temperatures ranging from 6000 K (H(2)) to 20,000 K (He) and a surface of emission whose radius ranges from 0.1 microm (He) to 0.4 microm (Xe) . The state of matter that would admit photon-matter equilibrium under such conditions is a mystery.

Cavitation luminescence in a water hammer: Upscaling sonoluminescence

Oscillatory acceleration and deceleration of a column of water leads to a pipe hammer as well as cavitation. With a small amount of xenon gas dissolved in the water, we can detect a stream of predominantly ultraviolet subnanosecond flashes of light which are attributed to collapsing bubbles. The observed emission can exceed 108 photons for a single collapse and has a peak power over 0.4 W.

Simultaneous measurement of triboelectrification and triboluminescence of crystalline materials

Triboelectrification has been studied for over 2500 years, yet there is still a lack of fundamental understanding as to its origin. Given its utility in areas such as xerography, powder spray painting, and energy harvesting, many devices have been made to investigate triboelectrification at many length-scales, though few seek to additionally make use of triboluminescence: the emission of electromagnetic radiation immediately following a charge separation event. As devices for measuring triboelectrification became smaller and smaller, now measuring down to the atomic scale with atomic force microscope based designs, an appreciation for the collective and multi-scale nature of triboelectrification has perhaps abated. Consider that the energy required to move a unit charge is very large compared to a van der Waals interaction, yet peeling Scotch tape (whose adhesion is derived from van der Waals forces) can provide strong enough energy-focusing to generate X-ray emission. This paper presents a device to press approximately cm-sized materials together in a vacuum, with in situ alignment. Residual surface charge, force, and position and X-ray, visible light, and RF emission are measured for single crystal samples. Charge is therefore tracked throughout the charging and discharging processes, resulting in a more complete picture of triboelectrification, with controllable and measurable environmental influence. Macroscale charging is directly measured, whilst triboluminescence, originating in atomic-scale processes, probes the microscale. The apparatus was built with the goal of obtaining an ab initio-level explanation of triboelectrification for well-defined materials, at the micro- and macro-scale, which has eluded scientists for millennia.

A novel technique to produce x-rays for XRF, medical, and scientific purposes

A long-standing mystery in science is the process whereby charge spontaneously exchanges between different materials that are brought into contact. After thousands of years of study there is no ab initio theory of tribocharging. As such it is an area of R&D that is not yet tethered to the first principles of physics and is wide open for new inventions. In 2008, Camara et al at UCLA discovered that tribocharging in a moderate vacuum could be used to take X-ray images. Since then, we have improved the X-ray output by 6 orders of magnitude and controlled the emission for use in a commercial product. Here we present an overview of this technology for use in X-ray fluorescence and X-ray imaging.

A triboelectric closed loop band system for the generation of x-rays

X-ray have been commercially produced using the same basic design since their discovery by Wilhelm Roentgen in 1895, for which he was awarded the first Nobel prize in physics. This technology requires high voltage elements, ultra high vacuum tubes, and high voltage electronics. The vacuum and high voltage drive up the price of x-ray technology and in order to bring down the cost, a brand new way to produce x-rays is needed. In 2008 Carlos Camara, Juan Escobar, Jonathan R. Hird, and Seth Putterman1 discovered that by pealing scotch tape in a vacuum you could create enough x-rays to take an x-ray radiograph of a finger. This lead to the formation of Tribogenics and the development of the rod and band x-ray architecture.

Experimental analysis of blackbody emission from sonoluminescence in sulfuric acid

The spectrum of emission from a single xenon bubble in concentrated sulfuric acid driven at 30 kHz is an excellent fit to Planck’s law with a surface temperature of 8000 K. The measured flash width and emission radius are also consistent with blackbody emission. In this study the only fitting parameter available is the temperature [Phys. Rev. Lett. 95, (2005)]. [Research funded by DARPA.]

Attempts to observe bubble fusion in deuterated acetone

A pulsed neutron generator (PNG) and a radioactive source have been used to seed cavitation at dynamic pressures ranging up to 35 atm. No fusion signal above background has been observed. Our upper bound is less than 1E−4 of the signal claimed by R. P. Taleyarkhan et al. [Phys. Rev. Lett. 96, (2006)]. Reasons for the failure of bubble fusion in acetone and future directions will be discussed. [Research funded by DARPA. We thank Brian Kappus for valuable discussions.]

Attempts to observe sonoluminescence from a single bubble driven at 10 MHz

A 10‐MHz spherical transducer array is used to generate cavitation at its focus with a field of ∼100 atm. Sonoluminescence as well as light‐scattering measurements will be discussed. [Research funded by DARPA.]

Why seek fusion from cavitation: Molecular dynamic simulations and a detector capable of time correlated single neutron counting

The blackbody spectra, and similar sonoluminescence intensities of He and Xe bubbles suggest that the interior of a sonoluminescing bubble is highly stressed and dense. Molecular dynamic simulations indicate interior temperatures which are enhanced by thermal conduction and can approach 1 MK. Furthermore the gas passes through states where the mean free path is larger than the distance over which temperature varies and so calls into question the value of theories based on hydrodynamics. To search for rare fusion events a neutron detector with 25% total discriminated quantum efficiency has been built. It can time stamp neutron arrival and sonoluminescence to better than 1 ns and record tracks on the fly. [Work supported by DARPA.]